Heretofore, it has been customary to mount an inducer on the distal end of the shaft of a pump for improving the suction capability of the pump. For example, an inducer disposed upstream of a centrifugal main impeller comprises an axial-flow or mixed-flow impeller which has configurational characteristics in that it has less blades and a longer blade length than ordinary impellers. The inducer is disposed upstream of the main impeller with its rotational axis aligned with the main impeller, and is rotated by the shaft at the same rotational speed as the main impeller.
Conventional inducers have blades designed to be of a helical shape. In the cross-sectional shape of blades, the tip, hub, and shaft center are positioned in line. According to a conventional process of designing inducers, a blade angle is designed only along the tip, and a blade angle is determined along the hub by helical conditions. The tip blade angle on the blade leading edge of a conventional inducer is designed to be greater than an inlet flow angle which is calculated from an axial inflow velocity of the flow in the inlet at a designed flow rate and a circumferential blade speed. The differential angle between the blade angle along the tip on the blade leading edge and the inlet flow angle is referred to as an incidence angle. The incidence angle is normally designed to be in a range from 35% to 50% of the blade angle on the blade leading edge. A blade angle from the inlet (leading edge) to the outlet (trailing edge) of the tip of the inducer is designed to be constant or to increase stepwise, linearly, or quadratically in order to meet a head required for the inducer.
When an inducer thus shaped is mounted in place, even if the pressure upstream of the inlet of the blades, i.e., the pressure of a fluid in an upstream region of the pump impeller, drops locally to a level that is equal to or lower than a saturated vapor pressure, thereby causing cavitation, a flow passage following a throat of the inducer is prevented from being closed by the cavitation, and the pressure of the liquid can be increased though the cavitation is developed. With the inducer disposed upstream of the main impeller, the suction capability of the pump can be improved as compared to a case where a centrifugal main impeller were used alone, and the pump can operate at a higher speed and can be smaller in size.
However, as described above, since the tip blade angle on the blade leading edge of a conventional inducer is designed to have an incidence angle to the flow in the inlet at a designed flow rate, and to be shaped such that a distribution of tip blade angles from the inlet to the outlet is constant or increases. Therefore, loads concentrate in the vicinity of the inlet of the inducer, tending to develop a reverse flow at the inlet. If the pump is operated in a partial flow rate range which is lower than the designed flow rate, then since the incidence angle at the inlet of the inducer becomes larger, the reverse flow developed at the inlet also becomes larger in scale. If a reverse flow is developed at the inlet while cavitation is being produced, the cavitation interferes with an upstream component, which tends to be damaged by the impact pressure of the cavitation.
Furthermore, the cavitation is generated and eliminated repeatedly at a low frequency within the reverse flow at the inlet, causing the pump to vibrate greatly in its entirety. In pumps for liquid hydrogen, the thermodynamic effect of hydrogen which acts to improve the suction capability is reduced by the reverse flow at the inlet, resulting in a reduction in the suction capability of the pump.
In view of the above drawbacks, it is of practical importance to design an inducer capable of suppressing the occurrence of a reverse flow at the inlet. Heretofore, attempts have been made to improve the blade angle, blade length, number of blades, and blade tip shape of inducers in order to satisfy the suction capability and a required head. However, efforts have not been made so far to improve the blade shape of inducers for suppressing a reverse flow at the inlet. At present, consequently, there have not yet been developed inducers for suppressing a reverse flow at the inlet while satisfying a required head and the suction capability.